Many precious metal deposits currently processed throughout the world are sulfidic in nature, and present challenges in the extraction and recovery of the contained gold. These deposits are not amenable to gold recovery by direct cyanidation as the gold is finely disseminated in the sulfide mineral crystal structure or as a solid solution that may be inaccessible to lixiviants. The cost of size reduction required to liberate the sulfide refractory gold is often prohibitive, and, in the case of gold disseminated as a solid solution, ineffective.
The preferred treatment, to liberate the gold is to oxidize the sulfides. Common methods of oxidation include bio-oxidation, roasting, atmospheric leaching, alkaline pressure oxidation (as in the process disclosed in Mason et al., U.S. Pat. No. 4,552,589), or acidic pressure oxidation (as disclosed in Thomas et al. U.S. Pat. No. 5,071,477). Roasting may not be suitable or economical for ores that contain low levels of sulfide and high levels of carbonates, because the roasting temperature cannot be maintained by the heat of the reaction.
In addition to the sulfide refractory nature of gold deposits, ores may also contain an active carbonaceous component, which has the ability to adsorb, or preg-rob, gold from the cyanide leach solutions thereby reducing recovery. This adsorption affinity of carbonaceous material forms the basis of the Carbon in Leach (CIL) and Carbon in Pulp (CIP) processes, which employ activated carbon granules to recover gold from gold cyanide solutions. The size and ruggedness of these commercially produced granules are such that they can be easily separated from the leach or pulp after adsorption has occurred by screening. Indigenous carbon competes with the commercially supplied graphite for the adsorption of gold, and is too small to be selectively removed by screening, and, therefore, the gold the indigenous carbon adsorbs is not recovered. An additional problem in recovering gold from highly carbonaceous ores is that a significant quantity of the gold may have been adsorbed onto carbon during formation of the mineral deposit. Cyanide has shown varying degrees of success in leaching gold locked in carbonaceous material.
Several strategies have been developed to reduce the preg-robbing.
The addition of blanking agents such as Kerosene, fuel oil, and RV-2 (para nitro benzol azo salicylic acid) selectively adsorb onto the surface of activated carbon in carbonaceous ores, thereby deactivating some of the preg-robbing character.
Carbonaceous matter can also be destroyed by roasting. This is the current industry standard for simultaneously destroying carbonaceous matter and oxidizing the sulfide minerals in refractory carbonaceous gold ores. This process is generally, but not always, successful and will depend upon the roasting temperature. Very high temperatures are often required to combust the graphitic carbon. Roasting plants operate in a narrow range of temperature tolerance. Below optimum temperature, the carbon in the ore is not oxidized and remains actively preg-robbing. Above the optimum temperature, the gold in the ore becomes increasingly less amenable to cyanidation or other extraction techniques.
In some cases, pressure oxidation can partially deactivate the indigenous carbon, but is not sufficient for highly preg-robbing ores. Processes such as that described in U.S. Pat. No. 5,364,453 employ flotation in which native and supplied carbon of cyanide leach residues are recovered by flotation. A disadvantage of this method is the size differential between the native carbon and commercial carbon. This would result in poor flotation behavior of the fine carbon.
Flotation of the carbonaceous component of double refractory ores prior to sulfide oxidation has proven to be inefficient, because ultra fine grinding is required to effectively separate the carbonaceous material from sulfide grains. Ultra fine grinding adds to the over all process costs and, depending on the ore's mineralogy, may not be fully effective at liberating the carbonaceous component from the gold bearing sulfides. In low grade ores, the loss of the gold containing sulfide to the carbonaceous flotation concentrate would significantly degrade the grade of the feed material to a hydrometallurgical or bacterial oxidation process.